A wall anchor constituted essentially by upsetting and bending a single metal strip, is described in FR-B 2 640 704 and in EP-B 0 378 907. An embodiment in which it is intended to be used, in the manner of a wood bit of the type known by the name “center bit”, to drill a partition in which it is to be anchored. It is driven by rotation on itself about its axis with respect to the partition. For example a screwdriver, acting on a head of the screw, the screw rod of which is screwed into the screw nut shaft and the head is positioned so as to bear longitudinally on the collar of the head shaft of the wall anchor.
The teeth of known wall anchor are oriented parallel to its axis and have a respective cutting inclined edge. The screw rod of the screw is extended, longitudinally opposite to the screw head, by an axial point which forms, with respect to the screw nut shaft, a projection greater than that of the teeth in order to engage in the partition before the latter. This ensures fixing of the common axis of the wall anchor and of the screw when the teeth come into contact with the partition. The teeth progressively penetrate into the partition under the action of the rotation imparted to the wall anchor about the axis.
Such guiding, ensures fixing of the axis of the wall anchor and of the screw with respect to the partition during drilling of the latter by the teeth of the screw nut shaft. The presence of the point which extends the screw rod of the screw beyond the anchor constitutes a drawback in so far as it is necessary. In order to accommodate this point, to dispose it within or at the back of the partition, a much larger clearance is necessary in the direction of the common axis of the wall anchor and the screw. This is larger than the clearance necessary to accommodate the screw rod, devoid of such a point in the case when standard screws are used in association with the more conventional wall anchors. Standard screws are devoid of drilling teeth and are anchored in a hole previously made in the partition by a conventional drill bit, independent of the wall anchor.
Another drawback of the necessary presence of the guide point in the extension of the screw rod of the screw lies in the impossibility of using a wall anchor produced according to the teachings of FR-B 2 640 704 and EP-B 0 378 907 in association with standard screws. The screw rod is devoid of any guide point if it is desired to benefit from the possibility of drilling the partition by the wall anchor itself. The use of the known wall anchor requires, in effect, the manufacture and holding in stock, specifically adapted screws equipped with the guide point. This creates constraints and additional manufacturing and storage costs. Also, screws which are specifically adapted require more material and are bulkier than the standard screws with identical dimensions of the actual screw rod.